windows 7 64bit on a pcie 2.0 vs windows 8 64bit on a pcie 3.0 will be about a 2-3% difference at best. this test has been done many times. its negligible and 3% will not make or break a gaming experience.
many many people are running p67/z68 mobos that have 2.0 pcie and running sli/cf even at 8x lane speed without much slowdown.
If you are not in the market to buy a new cpu then there is no point in changing mobos for the pcie 3.0, just put that money into a stronger gpu